Concentrated Solar Power

Introduction

Solar-thermal power plants convert solar radiation into heat and electricity. Reflectors (mirrors) are used to focus the sunlight onto absorbers, in which a carrier fluid or other medium is heated. This fluid serves to generate steam to drive a turbine, as in a conventional power plant. Widespread technologies include, for example, parabolic-trough power plants, solar tower power plants and dish-Stirling solar power systems.

Solar-thermal power plants require high levels of solar radiation. The most suitable locations for these are therefore regions in and around the Earth’s “sun belt” – up to 35 degrees latitude on either side of the equator. In 2016, the total installed capacity of solar-thermal power plants across the globe was approximately 5 GW, representing an almost four-fold increase on 2010 (see IRENA). Used in combination with a thermal storage solution, solar-thermal power plants are a cost-efficient means of providing large quantities of power and heat around the clock.

German companies have many years of experience with this technology. They are among the leading suppliers and service providers in the world for mirrors, absorbers, measuring instruments and management systems.

Solar-thermal power plants may differ in terms of the collector technology used: there are parabolic-trough power plants, tower power plants and dish-Stirling solar power systems.

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In parabolic-trough power plants, sunlight is focused on an absorber with selective coating. With the heat thus produced, a heat transfer medium such as heat-transfer oil is used to generate steam at temperatures of 400° C and over. The concentrating mirror elements are curved in the shape of a parabola (parabolic trough concentrating collectors) or consist of individual segments of flat mirrors (Fresnel collectors).

In solar tower power plants, sunlight is focused by meansof an array of biaxial sun tracking flat mirrors, or heliostats, onto a relatively small absorber located on a solar tower. Temperatures of over 1,000° C are achieved as a result of the high concentration of solar radiation, thus enabling highly efficient two-stage energy conversion. Today, there are several different technological approaches based on various heat transfer mediums such as air, water, steam or molten salt and heat exchangers such as shell and tube heat exchangers, atmospheric or pressurised volumetric structures.

Heat exchangers are used in many ways to transfer heat between two mediums.

In many heat exchangers, a hot and cold medium are streamed past a common heat transfer surface simultaneously. The heat flow transferred through these heat transfer surfaces varies according to the heat transfer coefficient of the heat exchanger, the size of the heat transfer surface and the mean temperature differential between the two mediums.

There are various types of heat exchangers, depending on the design and mode of operation. These include pipe bundle heat exchangers, plate heat exchangers, double-pipe heat exchangers, lamella heat exchangers, fin-tube heat exchangers, heat pipe heat exchangers, spiral heat exchangers and rotation heat exchangers. Heat exchangers have a broad area of application. An important area is power plant technology such as CSP power plants. In this case, the heat exchanger transfers the heat from the cycle taking up the heat through sunlight, which could, for example, run on thermal oil, to another cycle running on water, which drives the electricity-generating turbine.

In Dish Stirling solar power systems, the working gas of a Stirling engine, such as hydrogen or helium, is heated to a temperature of up to 900° C by a biaxial sun-tracking reflector to allow high electric efficiencies of around 30 percent. Dish Stirling solar power systems with a power output of between 10 and 50 kW are especially suited to decentralised applications.

The energy conversion process particular to all solar-thermal power plants allows the use of thermal stores or co-firing of fossil or biogenic fuels to make power plant operation more flexible. This variant is also known as hybrid operation. The electricity production thus enabled at peak load times or around the clock can greatly boost the profitability of the power plants.

Solar gas-steam-turbine (combined cycle) power plants are considered to be efficient solar power plants. For a high level of efficiency when converting solar radiation into electricity, the radiation must be directly coupled into the gas turbine.

In solar hybrid combined cycle plants, air is heated to drive the gas turbine, in part by solar radiation, in part by natural gas. Solar gas-steam-turbine power plants comprise heliostats, a solar tower, receiver and a combined cycle power plant component. After being concentrated about 1,000 times by the array of heliostats, the solar radiation is absorbed in receivers. Here, the air is intensely heated. The higher the air temperature achieved, the less natural gas is required as fuel to further heat the air to the necessary turbine entry temperature.